keychain command

I‘m using a Mac OS X and combination of ssh-agent+ssh-add to adds RSA or DSA identities to the authentication agent. ssh-agent provides me a secure way of storing the private key. However, I’d like to expire identities added to the agent within half an hour. How do I set lifetime of identities added to the agent under Unix / Linux / BSD / Apple OS X operating systems?

We establish connections to remote systems without supplying a password, however I do not want to store my password less keys ( passphrase-free keys) on my servers. ssh-agent, takes care of keys with passphase, which allowing me to easily have ssh-agent process per system per login session. How do I dramatically reduces the number of times I’ve to punch my passphrase from once per new login session to once every time my local server is rebooted? How do I use keychain utility for all my backup scripts for secure passwordless login?